1. Field of the Invention
The present invention relates to a silver and silver alloy plating bath. The present invention provides a bath with excellent stability over an extended time. With regard to silver alloy plating baths, the present invention provides a safe, non-cyanide bath, which can reliably codeposit silver and another metal.
2. Background Information
In general, silver readily forms an insoluble salt with various compounds. As a result, it is difficult to dissolve silver in a plating bath in a manner that is stable over an extended time. Decomposition of the bath and deposition of silver occurs readily. Furthermore, silver is an electrochemically noble metal, and as a result, alloy plating with other metals is difficult. Because of this, there are limitations on the types of silver plating baths that are practical. For example, in silver or silver-tin alloy plating baths, alkaline cyanide baths, containing various cyanide compounds, are known from the prior art.
However, cyanide compounds are extremely poisonous. Because special wastewater treatment is required, not only do treatment costs rise, but because it can only be used in the alkaline range, the types of companion metals are limited when conducting silver alloy plating. In addition, with alkaline baths, there are limitations on its uses, and in practical terms, these cyanide baths do not have adequate stability.
As a result, there is a need for development of a new silver or silver alloy plating bath, which is highly safe and in which silver can be dissolved in a stable manner over a wide pH range including strongly acidic pH's.
In Japanese Laid-Open Patent Publication Number 9-143786 (henceforth referred to as prior art 1), there is disclosed a non-cyanide silver plating bath which does not contain cyanide compounds. Prior art 1 is a silver plating bath, or a silver alloy plating bath, such as a silver-tin alloy, silver-copper alloy, silver-indium alloy, and the like, containing: thioglycol, thioglycolic acid, thiodiglycolic acid, beta-thiodiglycol, dibenzothiazole disulfide, 4,4′-thiobis (3-methyl-6-tert-butylphenol), or thiourea, and the like.
In the aforementioned prior art 1, by having the plating bath contain a specified sulfur-containing compound, such as thiodiglycolic acid, beta-thiodiglycol, dibenzothiazole disulfide, or thiourea, and the like, it is stated that the plate coating has a fineness similar to that achieved by cyanide plating baths of the prior art.
However, for example, with the above silver-tin alloy plating bath containing thiodiglycolic acid or beta-thiodiglycol and the like, in reality, there is often decomposition of the bath and deposition of silver in 2-4 weeks. As an electric plating bath for long-term, continuous usage, there are practical problems in its stability over an extended time.
Furthermore, when current density conditions are changed, the rate of codeposition of silver can fluctuate. If plating is conducted at high current densities, there are problems with burning or dendrites occurring on the electrodeposition coating. In addition, there are other problems, such as the substitution deposition of silver with respect to the plating substrate of copper or copper alloy and the like (in other words, deposition due to chemical substitution action based on oxidation-reduction electric potentials), or further substitution deposition of silver on top of the deposited silver alloy coating. As a result, the silver or silver alloy plating coating does not achieve a fine and high-quality outer appearance.
Using compounds such as thiodiglycolic acid and beta-thiodiglycol and the likedisclosed in prior art 1 as the starting point, the present invention has the technical objective of developing a stable, non-cyanide silver or silver alloy plating bath which contains compounds different from these.
With regard to the stability of Lewis acid-base complexes, general and qualitative definitions for hard and soft acids and bases are known (in other words, the HSAB principle), (refer to “Application of hard, soft, acid, base definitions to organic chemistry, “Yuuki gousei kagaku vol. 33 number 11 (1975)). For example, a base with a high electronegativity, a low polarity, and with a property of strongly holding its atomic valency electron is said to be a hard base. Conversely, a base with a low electronegativity, a high polarity, and with a property of holding the atomic valency electron relatively weakly is said to be a soft base. By coordinating a hard base to a hard acid, a more stable complex is formed. Furthermore, by coordinating a soft base to a soft acid, a more stable complex is formed.
Because silver ion, which has properties of a Lewis acid, can be classified as a soft acid, the present inventors believed that a soft base, which can combine easily with a soft acid, could be effectively used in order to stabilize the silver salt in a plating bath.
In the prior art 1, sulfide compounds such as thiodiglycolic acid, beta-thiodiglycol, dibenzothiazole disulfide, 4,4′-thiobis (3-methyl-6-tert-butylphenol), and the like are used. Thiourea is known as a chelating agent of silver (also disclosed in the aforementioned prior art 1), Taking these into consideration and based on the HSAB principle, intensive research was conducted on the behavior of various soft bases in silver or various silver alloy plating baths.